I distinguish paradoxes and hypodoxes among the conundrums of timetravel. I introduce ‘hypodoxes’ as a term for seemingly consistent conundrums that seem to be related to various paradoxes, as the Truth-teller is related to the Liar. In this article, I briefly compare paradoxes and hypodoxes of timetravel with Liar paradoxes and Truth-teller hypodoxes. I also discuss Lewis’ treatment of timetravel paradoxes, which I characterise as a Laissez Faire (...) theory of timetravel. Timetravel paradoxes are impossible according to Laissez Faire theories, while it seems hypodoxes are possible. (shrink)

Thinking about timetravel is an entertaining way to explore how to understand time and its location in the broad conceptual landscape that includes causation, fate, action, possibility, experience, and reality. It is uncontroversial that timetravel towards the future exists, and timetravel to the past is generally recognized as permitted by Einstein’s general theory of relativity, though no one knows yet whether nature truly allows it. Coherent timetravel stories (...) have added flair to traditional debates over the metaphysical status of the past, the reality of temporal passage, and the existence of free will. Moreover, plausible models of timetravel and time machines can be used to investigate the subtle relation between space-time structure and causality. -/- It surveys some philosophical issues concerning timetravel and should serves as a quick introduction. It includes a new and improved way to define a time machine. (shrink)

In the present paper, I offer a new argument to show that presentism about time is incompatible with timetravel. Timetravel requires leaving the present, which, under presentism, contains all of reality. Therefore to leave the present moment is to leave reality entirely; i.e. to go out of existence. Presentist “timetravel” is therefore best seen as a form of suicide, not as a mode of transportation. Eternalists about time do not (...) face the same difficulty, and timetravel is compossible with eternalism. (shrink)

Patients with damage to the ventromedial prefrontal cortex (VMPFC) are often described as having impaired ability for planning and decision making despite retaining intact capacities for explicit reasoning. The somatic marker hypothesis is that the VMPFC associates implicitly represented affective information with explicit representations of actions or outcomes. Consequently, when the VMPFC is damaged explicit reasoning is no longer scaffolded by affective information, leading to characteristic deficits. These deficits are exemplified in performance on the Iowa Gambling Task (IGT) in which (...) subjects with VMPFC perform significantly worse than neurotypicals in a task which requires them learn from rewarding and punishing experience to make decisions. The somatic marker theory adopts a canonical theory of emotion, in which emotions function as part of a valencing system, to explain the role of affective processes. The first part of the paper argues against this canonical account. The second part provides a different account of the role of the role of the VMPFC in decision-making which does not depend on the canonical account of emotion. Together the first and second parts of the paper provide the basis for a different interpretation of results on the Iowa Gambling Task (IGT). In fact the IGT may be probing a deficit in what has been called mental timetravel: the ability to access and use information from previous experience and imaginatively rehearse future experiences as part of the process of deliberation. (shrink)

In a dynamic world, mechanisms allowing prediction of future situations can provide a selective advantage. We suggest that memory systems differ in the degree of flexibility they offer for anticipatory behavior and put forward a corresponding taxonomy of prospection. The adaptive advantage of any memory system can only lie in what it contributes for future survival. The most flexible is episodic memory, which we suggest is part of a more general faculty of mental timetravel that allows us (...) not only to go back in time, but also to foresee, plan, and shape virtually any specific future event. We review comparative studies and find that, in spite of increased research in the area, there is as yet no convincing evidence for mental timetravel in nonhuman animals. We submit that mental timetravel is not an encapsulated cognitive system, but instead comprises several subsidiary mechanisms. A theater metaphor serves as an analogy for the kind of mechanisms required for effective mental timetravel. We propose that future research should consider these mechanisms in addition to direct evidence of future-directed action. We maintain that the emergence of mental timetravel in evolution was a crucial step towards our current success. (shrink)

Here I defend the compatibility of presentism and timetravel against a few objections. Keller and Nelson argue that, if presentism is at all plausible, presentism and timetravel are as compatible as eternalism and timetravel. But Miller and Sider are not convinced. I reply that for their concerns to have merit, Miller and Sider must assume presentists are committed to positions they need not be; I explain why presentists are not so committed (...) and, in the process, defend Keller and Nelson’s position that there is no roadblock to presentist timetravel that does not also apply to eternalist timetravel. (shrink)

This paper examines various philosophical arguments to do with timetravel. It argues that timetravel has not been shown to be logically impossible. It then considers whether timetravel would give rise to improbable strings of coincidences, or closed causal loops. Finally, it considers whether we could ever be justified in believing someone who claimed to be a time traveller, or whether we would always be more justified in believing that the claimant (...) was either deluded or trying to deceive us. For this last issue the Terry Gilliam film ‘Twelve Monkeys’ is used as an example. (shrink)

Combining timetravel with certain kinds of supertask, this paper proposes a novel model for Hell. Temporally-closed spacetimes allow otherwise impossible opportunities for material kinds of damnation and reveal surprising limitations on metaphysical objections to Hell. Prima facie, eternal damnation requires either infinite amounts of time or time for the damned to speed-up arbitrarily. However, spatiotemporally finite ‘timetravel’ universes can host unending personal torment for infinitely many physical beings, while keeping fixed finite limits (...) on rates of temporal passage. Such ‘Hilbert's Inferno’ spacetimes suggest neither materialism nor the finitude of time and space need forbid Hell. A material Hell can be spatiotemporally finite yet eternal for its inhabitants. Hilbert's Inferno also sheds light on Hell's location and accessibility, and shows that some spacetimes are intrinsically better suited to punishment than reward. (shrink)

Suppose that you travel back in time to talk to your younger self in order to tell her that she (you) should have done some things in her (your) life differently. Of course, you will not be able to make this plan work, we know that from the many versions of 'the grandfather paradox' that populate the philosophical literature about timetravel. What will be my centre of interest in this paper is the conversation between you (...) and ... you – i.e. the older you that travelled back in time and the younger you, when you first meet. As we shall see, given this situation, endurantists will have to endorse a strange consequence of their view : you will turn out to be a universal while your properties will turn out to be particulars. (shrink)

This paper is an enquiry into the logical, metaphysical, and physical possibility of timetravel understood in the sense of the existence of closed worldlines that can be traced out by physical objects. We argue that none of the purported paradoxes rule out timetravel either on grounds of logic or metaphysics. More relevantly, modern spacetime theories such as general relativity seem to permit models that feature closed worldlines. We discuss, in the context of Gödel's infamous (...) argument for the ideality of time based on his eponymous spacetime, what this apparent physical possibility of timetravel means. Furthermore, we review the recent literature on so-called time machines, i.e., of devices that produce closed worldlines where none would have existed otherwise. Finally, we investigate what the implications of the quantum behaviour of matter for the possibility of timetravel might be and explicate in what sense timetravel might be possible according to leading contenders for full quantum theories of gravity such as string theory and loop quantum gravity. (shrink)

Talk about timetravel is puzzling even if it isn't obviously contradictory. Philosophers however are divided about whether timetravel involves empirical paradox or some deeper metaphysical incoherence. It is suggested that timetravel requires a Parmenidean four-dimensionalist metaphysical conception of the world in time. The possibility of timetravel is addressed (mainly) from within a Parmenidean metaphysical framework, which is accepted by David Lewis in his defence of the coherence of (...)timetravel. It is argued that timetravel raises formidable difficulties which are not satisfactorily resolved by Lewis's ingenious defence of timetravel. Objections to timetravel considered include: (1) travel to other times is impossible because there is nowhere (or “nowhen”) to go to; (2) the problem that upon setting out on a journey to the past a time machine will collide with itself; (3) timetravel generates a mysterious temporal dualism between experiential time and physical time; (4) travel to the past permits reverse causation, raising the possibility of causal loops and attendant problems arising, for example, from the prospect of empirical contradiction and the possibility of someone being one of their ancestors. (shrink)

This idea of timetravel has long given philosophers difficulties. Most recently, in his paper ‘Troubles with TimeTravel’ William Grey presents a number of objections to timetravel, some well known in the philosophical literature, others quite novel. In particular Grey's ‘no destinations’ and ‘double occupation’ objections I take to be original, while what I will call the ‘times paradox’ and the ‘possibility restriction argument’ are versions of well known objections. I show how (...) each of these can be answered, thereby defending the plausibility of timetravel. (shrink)

In no possible world does a time traveler succeed in killing herearlier self before she ever enters a time machine. So if many,many time travelers went back in time trying to kill theirunprotected former selves, the time travelers would fail inmany strange, coincidental ways, slipping on bananapeels, killing the wrong victim, and so on. Such cases producedoubts about timetravel. How could ``coincidences'' beguaranteed to happen? And wouldn't the certainty of coincidentalfailure imply that (...)time travelers are not free to killtheir earlier selves? But if so, what would inhibit theirfreedom? Despite initial appearances, these and other doubtsmay be answered: the possibility of timetravel survives yetanother challenge. (shrink)

In this paper I consider two objections raised by Nick Smith (1997) to an argument against the probability of timetravel given by Paul Horwich (1995, 1987). Horwich argues that timetravel leads to inexplicable and improbable coincidences. I argue that one of Smith's objections fails, but that another is correct. I also consider an instructive way to defend Horwich's argument against the second of Smith's objections, but show that it too fails. I conclude that unless (...) there is something faulty in the conception of explanation implicit in Horwich's argument, timetravel presents us with nothing that is inexplicable. (shrink)

This paper argues that the most famous objection to backward timetravel can carry no weight. In its classic form, the objection is that backward timetravel entails the occurrence of impossible things, such as auto-infanticide—and hence is itself impossible. David Lewis has rebutted the classic version of the objection: auto-infanticide is prevented by coincidences, such as time travellers slipping on banana peels as they attempt to murder their younger selves. I focus on Paul Horwich‘s (...) more recent version of the objection, according to which backward timetravel entails not impossible things, but improbable ones—such as the string of slips on banana peels that would be required to stop a determined auto-infanticidal maniac from murdering her younger self—and hence is itself highly improbable. I argue that backward timetravel does not entail unusual numbers of coincidences; and that, even if it did, that would not render its occurrence unlikely. (shrink)

Existing puzzles about coinciding objects can be divided into two types, corresponding to the manner in which they bear upon the endurantism v. perdurantism debate. (Endurantism is the view that material objects lack temporal extent and persist through time by being wholly present at each moment of their careers. Perdurantism is the opposing view that material objects persist by being temporally extended and having different temporal parts located at different times.) Puzzles of the first type, which involve (...) temporary spatial co-location, can be solved simply by abandoning endurantism in favor of perdurantism, whereas those of the second type, which involve career-long spatial co-location, remain equally puzzling on both views. I show that the possibility of backward timetravel (either via discontinuous jumps or via closed timelike curves) would give rise to a new type of puzzle. The new puzzles confront perdurantists and can be solved just by shifting to endurantism. (shrink)

Humans have long been fascinated by the idea of visiting the past and of seeing what the future will bring. Timetravel has been one of the most popular themes of science fiction. Most people have seen the TV series ‘Dr Who’ or ‘Quantum Leap’ or ‘Star Trek’. You’ve probably seen one of the ‘Back to the Future’ or ‘Terminator’ movies, or ‘Twelve Monkeys’. Timetravel narratives provide fascinating plots, which exercise our imaginations in ever so (...) many ways. But is the idea of travelling forward and backward in time pure fantasy—or can it be done? To be sure, not all timetravel scenarios are coherent. But we hope to persuade you that the most common objections to the very idea of timetravel have no real force. (shrink)

Timetravel has been a staple of science fiction. With the advent of general relativity it has been entertained by serious physicists. But, especially in the philosophy literature, there have been arguments that timetravel is inherently paradoxical. The most famous paradox is the grandfather paradox: you travel back in time and kill your grandfather, thereby preventing your own existence. To avoid inconsistency some circumstance will have to occur which makes you fail in this (...) attempt to kill your grandfather. Doesn't this require some implausible constraint on otherwise unrelated circumstances? We examine such worries in the context of modern physics. (shrink)

If (backward) timetravel is possible, presumably so is my shooting my younger self (YS); then apparently I can kill him – I can commit retrosuicide . But if I were to kill him I would not exist to shoot him, so how can I kill him? The standard solution to this paradox understands ability as compossibility with the relevant facts and points to an equivocation about which facts are relevant: my killing YS is compossible with his proximity (...) but not with his survival, so I can kill him if facts like his survival are irrelevant but I cannot if they are relevant. I identify a lacuna in this solution, namely its reliance without argument on the hidden assumption that my killing YS is possible : if it is impossible, it is not compossible with anything. I argue that this lacuna is important, and I sketch a different solution to the paradox. (shrink)

The paper first tries to explain how the possibility of "timetravel" arises in the Godel universe. It then goes on to discuss a technical problem conerning minimal acceleration requirements for timetravel. A theorem is stated and a conjecture posed. If the latter is correct, timetravel can be ruled out as a practical possibility in the Godel universe.

This paper demonstrates that John Wheeler and Richard Feynman's strategy for avoiding causal paradoxes threatened by backward causation and time-travel can be defeated by designing self-interacting mechanisms with a non-simple topological structure. Time-travel therefore requires constraints on the allowable data on space-like hypersurfaces. The nature and significance of these constraints is discussed.

The possibility of timetravel, as permitted in General Relativity, is responsible for constraining physical fields beyond what laws of nature would otherwise require. In the special case where timetravel is limited to a single object returning to the past and interacting with itself, consistency constraints can be avoided if the dynamics is continuous and the object's state space satisfies a certain topological requirement: that all null-homotopic mappings from the state-space to itself have some fixed (...) point. Where consistency constraints do exist, no new physics is needed to enforce them. One needs only to accept certain global topological constraints as laws, something that is reasonable in any case. (shrink)

The territory of timetravel has, from the days of H. G. Wells to the mid-1980's, been the exclusive province of writers of science fiction and fantasy. SF critics have even argued that timetravel stories are so scientifically unlikely that they should be considered fantasy, not science fiction.

There is a tension between the “growing block” account of time (closed past, open future) and the possibility of backwards timetravel. If Tim the time traveler can someday travel backwards through time, then he has (in a certain sense) already been. He might discover this fact before (in another sense) he goes. Hence a dilemma: it seems that either Tim’s future is determined in an odd way or cases of (temporary) ontic indeterminate identity (...) are possible. Either Tim cannot avoid heading for the past or he is only indeterminately that guy who appeared in the past with many of Tim’s memories. Determinacy in the past implies a degree of determinism in the future; indeterminism about the future seeps back to the past. (shrink)

Extending work of Wittgenstein, Lakoff and Johnson I suggest that it is the (spatial) metaphors we rely on in order to conceptualise time that provide an illusory space for time-travel-talk. For example, in the “Moving Time” spatialisation of time, “objects” move past the agent from the future to the past. The objects all move in the same direction – this is mapped to time always moving in the same direction. But then it is easy (...) to imagine suspending this rule, and asking why the objects should not start moving in the opposite direction. This is one way of generating the idea of time-travel “back” into the past. Time-travel-talk essentially involves the unaware projection of fragments of our time-talk – taken from powerful conceptual metaphors – onto the nature of reality itself. Understanding this dissolves away the charm and attractions of such talk. (shrink)

This paper argues that, in light of certain scenarios involving timetravel, Sider’s definition of ‘instantaneous temporal part’ cannot be accepted in conjunction with a semantic thesis that perdurantists often assume. I examine a rejoinder from Sider, as well as Thomson’s alternative definition of ‘instantaneous temporal part’, and show how neither helps. Given this, we should give up on the perdurantist semantic thesis. I end by recommending that, once we no longer accept such semantics, we should accept a (...) new set of definitions, which are superior in certain respects to Sider’s original set. (shrink)

The present paper offers a philosophical discussion of phenomena which in the empirical literature have recently been subsumed under the concept of ‘mental timetravel’. More precisely, the paper considers differences and similarities between two cases of ‘mental timetravel’, recollective memories (‘R-memories’) of past events on the one hand, and sensory imaginations (‘S-imaginations’) of future events on the other. It develops and defends the claim that, because a subject who R-remembers a past event is experientially (...) aware of a past particular event, while a subject who S-imagines a future event could not possibly be experientially aware of a future particular event, R-memories of past events and S-imaginations of future events are ultimately mental occurrences of two different kinds. (shrink)

The last few years have been good for time machines. Kip Thorne's renowned general relativity group at Caltech invented a new quantum gravitational approach to building a time gate, and, in an international collaboration, gave a plausible rebuttal of "grandfather paradox" arguments against timetravel. Another respected group suggested time machines that exploit quantum mechanical time uncertainty. The technical requirements for these suggestions exceed our present capabilities, but each new approach seems less onerous than (...) the last. There is hope yet that timetravel will eventually become possible, even cheap. (shrink)

In 1987, Roy Sorensen coined the term 'parahistory' to denote the study of genuinely anachronistic artefacts delivered by timetravel.¹ 'Parahistory' would thus stand to history rather as parapsychology is claimed to stand to psychology, i.e. the parahistorian would study historical data that were obtained through channels that orthodox science does not recognise. How might one establish credentials as a time traveller? What sort of evidence could a time-traveller point to in support of claims that would (...) presumably command a great deal of scepticism? While successful predictions might be one confirmatory tool, supporting evidence needn't take the form of predictions. (Although Sorensen offers an intriguing argument that a Humean about miracle-testimony would be obliged to reject any testimony to the occurrence of timetravel.) Perhaps artefacts that time travellers retrieve from other times (past or future) could usefully supplement testimonial or predictive evidence for the parahistorian. However, this paper will argue, any appeal to past-artefactual evidence for parahistorical claims faces a dilemma that threatens to undermine its value: appeals to past-artefact evidence must either collapse into appeals to predictive evidence or see their value diminished by such past artefacts' necessarily exhibiting contradictory indicators of age and period. (shrink)

I begin by sketching a theory about the semantics of verbs in event sentences, and the evidence on which that theory is based. In the second section, I discuss the evidence for extending that theory to state sentences, including copulative sentences with adjectives and nouns; the evidence for this extension of the theory is not very good. In the third section, I discuss new evidence based on considerations of talk about timetravel; that evidence is apparently quite good. (...) I conclude with a problem about formulating default knowledge. (shrink)

There is a growing interest in mental timetravel in cognitive psychology, neuroscience, developmental psychology, comparative psychology, and evolutionary psychology. Here we review current issues in each of these disciplines. To help move the debates forward we name and distinguish 15 key hypotheses about mental timetravel. We argue that foresight has for too long lived in the shadows of research on memory and call for further research efforts.

The role of time in episodic memory and mental timetravel is considered in light of findings on humans' temporal memory and anticipation. Time is not integral or uniform in memory for the past or anticipation of the future. The commonalities of episodic memory and anticipation require further study.

We are not entirely satisfied with the evolutionary explanation provided by Suddendorf & Corballis (S&C) for why only humans should be capable of advanced mental timetravel. General social factors do not suffice, given that other primates are also highly social. We discuss the evolutionary mechanisms that have generated mental timetravel typical to humans, focusing on ecological factors.

The possibility of timetravel, as permitted in General Relativity, is responsible for constraining physical fields beyond what the laws normally require. In the special case where timetravel is limited to a single object returning to the past and interacting with itself, consistency constraints can be avoided if the dynamics is continuous and the object’s state space satisfies a certain topological requirement: that all null-homotopic mappings from the state-space to itself have some fixed point. Where (...) consistency constraints do exist, no new physics is needed to enforce them. All one needs for their accommodation is to grant a kind of law-like status to some kinds of global boundary conditions. (shrink)

Is timetravel just a confusing plot device deployed by science fiction authors and Hollywood filmmakers to amaze and amuse? Or might empirical data prompt a scientific hypothesis of timetravel? Structured on a fascinating dialogue involving ...

I have previously argued in a paper with Robson that a particular timetravel scenario favours perdurantism over endurantism on the grounds that endurantists must give up on the Weak Supplementation Principle. Smith has responded, arguing that the reasons we provided are insufficient to warrant this conclusion. This paper agrees with that conclusion (for slightly different reasons: that even the perdurantist has to give up on the Weak Supplementation Principle) but argues that the old argument can be supplanted (...) with a new one. (shrink)

According to Chomsky, creativity is a critical property of human language, particularly the aspect of ?the creative use of language? concerning the appropriateness to a situation. How language can be creative but appropriate to a situation is an unsolvable mystery from the Chomskyan point of view. We propose that language appropriateness can be explained by considering the role of the human capacity for Mental TimeTravel at its foundation, together with social and ecological intelligences within a triadic language-grounding (...) system. Our proposal is based on the change of perspective from the analysis of individual sentences to the flux of speech in which the temporal dimension of language is much more relevant. (shrink)

Mental timetravel requires the sense of a past and future self, which is lacking in the early years of life. Research on the development of autobiographical memory and development of self sheds light on the difference between memory in other animals and its cultural narrative basis in humans.

In this paper I offer a rejoinder to the criticisms raised by Jimmy Alfonso Licon in “No Suicide for Presentists: A Response to Hales.” I argue that Licon's concerns are misplaced, and that his hypothetical presentist time machine neither travels in time nor saves the life of the putative traveler. I conclude that sensible timetravel is still forbidden to presentists.

Although the mental timetravel (MTT) hypothesis provides a rich, conceptual framework, the absence of clear, empirically tractable, behavioural criteria for determining the capacity for MTT restricts its usefulness in comparative research. Examples of empirical criteria for evaluating MTT in animals are given. We also question the authors' evaluation of semantic foresight and their even-handedness in assessing human and nonhuman behaviour.